You're absolutely right that the rise of Trump represented as loss of control of the GOP "merchant elite" establishment to an insurgent right wing populist base, a control it has not been able to reassert. Trump even initially alienated elements of the old conservative intelligentsia (around, e.g., the National Review). But the rise of a figure like Trump was a very long time coming within the larger conservative movement, starting way back with the right wing opposition to Reagan as, ultimately, too bipartisan and too generally sunny in his view of America and American culture. Figures like Gingrich and Pat Buchanan tried to ride this right wing tiger, both ultimately unsuccessfully, relative to their oversized aspirations. They were were ultimately succeeded by G W Bush, arguably the last Reaganite GOP standard bearer, but one with some purchase on the movement right. After Bush did his time, the establishment denied the right wing of the party base a properly firebrand social conservative leader, offering up instead the erratic John McCain and the milquetoast Mitt Romney in succession. This, much of the base felt, lead directly to the election and re-election of Barack Obama, in many ways their worst nightmare (not least because his appeal created the possibility of reconstituting the old New Deal Democratic electoral bloc that left the GOP out of executive power for almost 40 years). And the GOP was genuinely in a very bad place post-Obama, with the Trump candidacy representing a lark not dissimilar to celebrity stunt candidacies of the past. When the establishment once again tried to ram a boring and weak candidate down their throat (remember when Jeb Bush was their guy?) they lost it and fell in wholesale with the thrillingly transgressive D. Trump.
But, three things:
1. Both the old merchant elite who used to control the GOP and the movement intelligentsia were not long at all in warming to Trump, particularly in the absence of ANY other figure within the party with legitimate national appeal. The alternative was another Democratic president with another 4-8 years to rebuild the old New Deal majority.
2. The conservative movement base may seem as though it lacks particular focus or vision and is only focused on immediate electoral success, but it has actually been active in some form for many decades on its two main political projects-- ending what's left of the New Deal and Great Society programs ("big government") and overthrowing the separation of church and state. It saw Trump's surprise success as an important opening-- one that it has so far made the most of. It is NOT only focused on short term electoral successes, even if its vision for the country is ultimately unrealizable. It's going to make life miserable for a lot of ordinary people for many years to come before this is all over.
3. The Democratic Party post-Bill Clinton was never going to do a thing to rebuild the old New Deal majority (by e.g. undoing Reagan era labor reforms and passing single-payer health care). Instead, it opened the door wide to right wing populism by offering American workers, many of whom only a few years before had been willing to take a chance on Barack Obama, absolutely nothing but the old lesser-evilism, neo-liberal bromides, and warmed over P.C. niceties. And it wasn't as though working people flocked to Trumpism (it was and continues to be largely a movement of "rich people from poor places"). But just enough ordinary working people in just the right places got sick enough of the Democrats and their politics of urban and suburban elite meritocracy to take a chance on a faux populist like Trump. Most continued to not vote or engage in politics at all (except for a small minority of engaged workers on the left of the Democratic Party). But it doesn't take much at all to shift the knife-edge balance of US federal partisan politics. Only a true mass mobilization of new voters could alter this reality, and neither party seems willing or able to undertake this challenge. Both remain vehicles for elite prerogatives in their respective ways, and the country will continue to suffer for it. This is how empires collapse, BTW. From within.